The primary structural component of a bicycle is the bicycle frame. Typically, the bicycle frame comprises an elongate top tube which is rigidly secured to and extends between a head tube of the bicycle and a seat tube of the bicycle. The head tube typically provides a structural base for the stem of the bicycle to which the handle bars are attached. The seat tube provides a base for a seat post which is generally telescopically received therewithin and to which is secured the saddle or seat of the bicycle. In typical bicycle frame construction, the seat tube includes a generally cylindrical axle-receiving bracket attached to the lower end thereof which is adapted to receive the bottom bracket axle. The bottom bracket axle typically extends between and interconnects the cranks to which are attached the pedals. Rigidly secured to and extending between the head tube and the cylindrical axle-receiving bracket is an elongate down tube.
In addition to the aforementioned structural components, rigidly secured to and extending rearwardly from the axle-receiving bracket are first and second chain stay members. Additionally, rigidly secured to and extending downwardly from the upper end of the seat tube are first and second seat stay members having lower ends which are rigidly secured to the back ends of the first and second chain stay members. Typically, the lower ends of the seat stay members and back ends of the chain stay members are interconnected in a manner adapted to receive the rear tire axle of the rear wheel. The head tube, seat tube, top tube, and down tube are typically secured to each other and to the axle-receiving bracket in a manner defining a main front triangle portion of the bicycle frame. The seat stay and chain stay members, when connected to the seat tube, axle-receiving bracket, and each other, typically define a back triangle portion of the bicycle frame.
The foregoing description generally represents the construction of conventional prior art bicycle frames. Typically, when such prior art frames are constructed, the aforementioned structural components are rigidly secured to one another through the use of welding or bracing techniques. Though this method of constructing the bicycle frame provides the resulting frame with structural integrity, the bicycle frame does not possess a suspension having shock absorbing characteristics. As will be recognized, the ride, comfort, and performance of the bicycle would be greatly enhanced if the bicycle frame were adapted to at least partially accommodate the shocks routinely encountered while riding the bicycle.
Though recent prior art bicycle frames include front and/or rear shock absorbing assemblies, such bicycle frames possess certain deficiencies which detract from their overall utility. In most prior art rear shock absorbing assemblies, the rear axle pivots about a single elevated pivot point when subjected to a shock force which generally results in the rear wheel axle moving upwardly in an arc rather than moving vertically upward in a substantially linear fashion.
Typically, if the rear wheel axle is caused to move arcuately due to the absorption of a bump or shock force by the rear tire, the bicycle frame will normally rise and fall a few times due to suspension oscillations after the obstacle or obstruction has been cleared by the rear tire. This bouncing action which occurs at a frequency attendant to the structure of the rear shock absorbing assembly will typically require the rear tire to speed up and slow down as it keeps up with the bicycle's constant velocity, since the wheel base of the bicycle is changing as the rear wheel axle moves arcuately back to its original position. This constant changing of the rear tire's angular velocity requires energy due to the effects on the rear tire's angular momentum, thus diminishing riding efficiency.
Further, the rear shock absorbing assemblies are typically mounted directly to the main front triangle portion of the bicycle frame, and are configured in a manner which results in the amount of rear wheel travel being greater or less than the amount of shock absorber travel when a shock force is applied to the rear wheel. In certain prior art rear shock-absorbing assemblies, less and less additional force is required to compress the shock absorber of the assembly for each equal increment in rear wheel movement due to the mechanical advantage of the shock absorber over the rear wheel decreasing throughout the rear wheel travel. This type of suspension wherein the wheel rate is regressive is generally undesirable due to the tendency of the shock absorber to "bottom-out". Other prior art rear shock absorbing assemblies are configured in a manner so as to achieve a progressive wheel rate wherein more and more additional force is needed to compress the shock absorber for each equal increment in rear wheel movement. Though a progressive wheel rate is more desirable than a regressive wheel rate, optimal performance of the bicycle is achieved with a flat wheel rate wherein the ratio of movement, i.e. the motion ratio, between the shock absorber and the rear wheel is constant throughout the range of vertical travel of the rear wheel.
In addition to the foregoing, the mounting of the shock absorber assembly to the main front triangle portion of the bicycle frame sometimes results in the force of the shock being transmitted directly to the main front triangle portion of the bicycle frame as bending moments or torque which adversely affects the overall smoothness of the bicycle ride. As such, a much more smooth and even ride would be obtained if the shock absorber assembly was not mounted directly to the main front triangle portion of the bicycle frame, and was configured to provide a flat rate of rear wheel travel.
The use of such a bicycle rear suspension system is described in more detail in U.S. Pat. No. 5,441,292 issued on Aug. 15, 1995 to Busby and entitled "Bicycle Rear Suspension System", the contents of which are hereby incorporated by reference.
Although bicycle frames utilized for such rear suspension systems have proven generally suitable for their intended purpose, they possess inherent deficiencies which detract from their overall desirability and effectiveness in the marketplace. As those skilled in the art will appreciate, such contemporary bicycle frames utilize pivot or flex joints which generally comprise a pivot pin which extends through apertures formed at the frame member ends to be so joined. Although such construction does provide flexing along a desired axis while mitigating flexing along other axes and also mitigates torsion of the two frame members relative to one another, such construction necessitates the forming of apertures at the frame ends to be joined. Each aperture must be machined into a solid member or lug which is generally welded to the tubular frame member. Typically, this is done to both framed members to be so joined, so as to define a clevis. Such construction methodology inherently requires precision tooling and skilled labor. Thus, flex joints formed according to such contemporary methodology are costly and comparatively difficult to fabricate. Such prior art joints also have an inherent need for maintenance such as cleaning and lubrication. Also, such prior art flex joints are susceptible to contamination, particularly since bicycles are commonly used in environments where they are exposed to various environmental contaminants such as dirt, sand, debris, etc.
As such, it is beneficial to provide a flex joint for a bicycle frame which facilitates flexing about a single desired axis while mitigating flexing about all other axes, as well as mitigating torsion of the interconnected frame members relative to one another, and which also does not require substantial maintenance, is inexpensive to manufacture, and which facilitates easy assembly of the bicycle frame.